The invention relates to an improved thermal switch which closes upon heating to a given temperature and which can safely withstand a temperature much higher than that of closure. Such switches are often used in high intensity discharge lamps wherein they are located in the interenvelope space between the inner arc tube of fused silica and the outer glass envelope.
In metal halide lamps in which the fill comprises mercury and the halides of various metals including sodium, electrolysis of the fused silica in the region between the starter and the adjacent main electrode is a problem. A solution thereto, widely adopted in the lamp industry, was provided by U.S. Pat. No. 3,226,597 -- Green. It consists in a bimetal switch located in the interenvelope space between the arc tube and the outer envelope, which short circuits the starter electrode to the adjacent main electrode after the lamp has heated up to operating temperature.
Metal halide lamps have generally utilized evacuated outer envelopes to minimize heat losses from the fused silica arc tube. The thermal switch is heated by conduction from the arc tube structure and by radiant energy, so that the switch temperature is determined by the lamp wattage and is substantially independent of the position or orientation in which the lamp is operated. A simple U-shaped switching element could be designed to accommodate the temperature excursions normally encountered over the operating wattage range of the lamps without exceeding the thermal and mechanical limitations of the commercially available bimetal materials. Such material, often termed thermostat metal, is a composite, usually in the form of strip or sheet, made up of two or more metallic layers of different coefficients of expansion permanently bonded together. A nickel-iron alloy is commonly used for the low expansion component, and a nickel-chrome steel alloy for the high expansion component. When the temperature is raised, the relative lengths of the two components change causing the material to curve or bend.
Various considerations including long term performance, processing economies whereby to avoid the cost of exhaust, and safety considerations in the case of physically large lamps, now make it desirable to provide a gas filling within the outer protective envelope. A consequence thereof is to introduce the variable of convective heating in lamp performance which has a strong effect on the operating temperature of the bimetal element. For practical reasons of ease of mounting and manufacturing, the bimetal element is normally located at the end of the arc tube which has the starter electrode. In gas filled lamps, the arc tube is arranged to have the starter electrode uppermost. Under these circumstances, convective heating of the bimetal switch during operation If greatly influenced by the operating position or orientation of the lamp from vertical to horizontal. Consequently the switch must function over a much greater range of temperature than previously when wattage variation alone was the governing factor.
In a bimetal switch which is normally closed and opens upon heating and wherein there is no restraint upon further deformation, overheating does not create any particular problem. However where the switch is normally open and closes upon heating in such fashion that contact closure imposes a restraint on movement of the bimetal, excessive temperature excursions may stress the bimetal material beyond its elastic limits resulting in permanent deformation of the switch. When the switch deforms, it may no longer close at the design closure temperature. A metal halide lamp wherein the switch fails to close may become inoperative as a result of electrolysis after a few hundred hours of operation.